1. Field of the Invention
This invention relates to the detection of the radioisotope carbon-14 using Accelerator Mass Spectrometry (AMS). Although limitations in scope are not intended, this invention has particular relevance to the fields of attomole (10.sup.-18 molar) detection of chemicals. Also neutron exposure monitoring around nuclear reactor and weapons production sites.
2. Description of Prior Art
Attomole and even zeptomole (10.sup.-21 mole) detection are becoming important in modern biochemistry, particularly with the introduction of separation procedures such as Capillary Electrophoresis, where the yield of a specific fraction of the defined material may be only picomoles. Carbon-14 is important as a label for tracking such small quantities. It has a unique detection signature and can be incorporated into specific ligands of an organic compound without modifying the compound's chemical behavior. However, historically, carbon-14's half life is so long that when decay methods are used for detection, attomole (10.sup.-18 mole) sensitivities are not available; the detection efficiency is such that it is barely possible to detect five femtomoles of carbon-14 in an idea sample.
During the last fifteen years the sensitivity for detecting carbon-14 labels has been enhanced by many orders of magnitude using the detection technique of accelerator mass spectrometry (AMS). Using AMS, individual carbon-14 atoms are detected directly using mass spectrometric methods rather than waiting for the associated radioactive decay. In practice, seven orders of magnitude increase in sensitivity become available. The principles of AMS have been described in detail by a number of authors including, for example U.S. Pat. No. 4,037,100 to K. H. Purser; K. H. Purser, H. E. Gove and A. E. Litherland, "Ultra-sensitive Particle Identification Systems Based Upon Electrostatic Accelerators", Nuclear Instruments and Methods, Volume 162, page 637 (1979), and by D. Elmore and F. M. Phillips "Accelerator Mass Spectrometry" in Science, Volume 236, page 543, (1987).
Examples of the potential applications of AMS detection of carbon-14 labels in biomedicine have been reviewed by J. S. Felton, K. W. Turteltaub, J. S. Vogel, R. Baithorne, B. L. Gledhill, J. R. Southon, M. W. Caffee, R. C. Finkel, D. E. Nelson, I. D. Proctor and J. C. Davis in Nuclear Instruments and Methods in Physics Research, Volume B52, page 517 (1990). These workers have demonstrated chemical detection limits of quantities as small as 300 zeptomoles. The high efficiency of detection is such that attomole quantities of molecules enriched in .sup.14 C to only 100 to 1000 times the activity of natural biological carbon, can provide clear signals.
Apart from the new frontiers that become available in biomedicine, using AMS, an important social consequence is that disposal becomes trivial for carbon-14 waste generated by such low-level experiments. Incineration of waste followed by stack gas dilution with "dead" carbon dioxide (from the simultaneous burning of ordinary fuel oil or dilution with tank CO.sub.2) can produce a released activity of carbon-14 that is at or even below the natural level of carbon-14 in the atmosphere.
3. Existing Size Limitation
Referring to FIG. 1, it can be seen that AMS is a type of tandem mass spectrometry in which the two mass spectrometers are separated by an electrostatic accelerator that provides a kinetic energy gain from the kilo-electron-volt energies (keV) used in classical mass spectroscopy to million-electron-volt energies (MeV). At MeV energies, nuclear detection techniques can be applied for counting and identifying individual ions, often with high efficiency and precision. The accelerator also effects a conversion from negative to multiply charged positive ions.
It is known that all molecular interferences can be eliminated by the Coulomb explosion process if carbon-14 atoms in charge states 3.sup.+ or above are selected by the second analysis stage of the instrument shown in FIG. 1. Hoffman, et al. in Nuclear Instruments and Methods, Volume B5, page 254, (1984) shows that to achieve a maximum yield of .sup.14 C.sup.3+ ions (the minimum positive ion charge state that guarantees molecule dissociation for the interfering molecular species relevant to carbon-14 detection), tandem voltages of 2-3 million volts are essential. Thus, the necessity for insulating such voltages sets the scale of size for the whole AMS instrument. Tandems employing terminal voltages of 3 MV are not small and their size tends to preclude installation within conventional biomedical or chemical laboratories.
As a method of avoiding the above 3+constraint, K. H. Purser in U.S. Pat. No. 4,973,841 describes a more compact apparatus based upon stripping to the 2.sup.+ charge state during the acceleration phase. To guarantee molecular background elimination, a second stripping is made to 3.sup.+ before final analysis and detection. At an energy of 800 keV, more than 50% of the carbon ions leave a foil in the 2.sup.+ charge state so that in this part of the apparatus the efficiency is acceptable. However, the succeeding requirement of U.S. Pat. No. 4,973,841 for a second charge changing transition to the 3.sup.+ charge state, reduces the detection efficiency by at least a factor of two below that of conventional 3 MV machines.